Effective operation of selective catalytic reduction (SCR) catalysts is of increasing interest to meet modern internal combustion engine emissions standards. The effectiveness of a typical SCR catalyst in removing oxides of nitrogen (NOx) emissions is sensitive to the temperature of the exhaust gas at the inlet to the SCR catalyst. Under cold start conditions and low temperature phases of transient cycles, an SCR catalyst may not reach effective reduction temperatures to reduce NOx to N2 even if ammonia is stored on the catalyst and reductant is available for dosing into the exhaust flow.
Typical diesel aftertreatment systems include a diesel oxidation catalyst (DOC) and a diesel particulate filter (DPF) in addition to the SCR catalyst. The DOC is responsible for oxidation of hydrocarbons (HC), carbon monoxide (CO), and nitric oxide (NO). The DPF can also convert NO to nitrogen dioxide (NO2) under certain operating conditions. The conversion of NO to NO2 upstream of the SCR catalyst can improve feed gas composition to the inlet of the SCR catalyst for efficient NOx conversion. However, in general, the DOC and DPF do not operate efficiently at low temperature conditions to convert NO to NO2 in an oxidation reaction. While thermal management of the exhaust flow can be provided to increase exhaust temperatures to an effective operating range, these techniques are often associated with a fuel economy penalty. Also, reductant injection upstream of the SCR catalyst in low temperature conditions can create reductant deposits. Reductant deposits can degrade the deNOx (NOx reduction) performance of the SCR catalyst, increase back pressure during engine operation, and decrease fuel economy due to the need for thermal management of the exhaust system to remove reductant deposits.
In order to reduce NOx emissions levels and improve aftertreatment system performance, improvements in aftertreatment designs that mitigate the adverse effects of low temperature exhaust conditions are needed. Accordingly, further technological developments in this area are desirable.